This invention relates generally to composite plastic molded articles with improved heat resistance and, specifically, to a composite phenolic brake piston with improved heat resistance. The brake piston of the invention is intended for use in the automotive and truck industry.
The invention is also applicable to composite molded articles that are useful in other applications that are exposed to high temperatures in the automotive, appliance, electrical, electronic and aerospace industries.
Automotive and truck disc brake pistons are currently manufactured from:
1. Steel (plated with nickel or chrome)
2. Phenolic resin molding compounds
3. Phenolic resin molding compounds provided with a metal cap.
Each material demonstrates a different performance in braking systems.
Steel pistons have high thermal conductivity that can result in high brake fluid temperature which is a safety concern. Phenolic pistons are thermal insulators that result in lower brake fluid temperatures. Phenolic pistons with metal caps provide better thermal insulation and lower fluid temperatures than metal pistons, but are not as good an insulator as all plastic pistons.
Phenolic disc brake pistons have heat resistance limits resulting in surface cracks or blisters from thermal shock on short term exposure to high temperatures. The phenolic piston can degrade from long term or repeated exposure to high temperatures. This is the result of oxidation of the phenolic material.
Phenolic pistons with a molded on metal cap as a heat shield have improved crack resistance compared to phenolic pistons without a metal cap. The long term heat resistance or oxidation resistance is also improved.
Weak points of the phenolic piston with the metal cap are:
1. The molded cap loosens on long term exposure to high temperatures due to the differential thermal expansion of the steel as compared to phenolic materials. Further, phenolic materials undergo post mold shrinkage. PA1 2. There is also a heat concentration at the interface of the metal cap with the outside surface of the phenolic piston. PA1 1. Low friction metal such as bronze. PA1 2. Phenolic molding compounds PA1 1. The molded articles of two similar phenolic molding compounds are simultaneously co-molded in one step. PA1 2. The phenolic sheet molding compound is shaped to the contour of the mold in the molding process PA1 3. There is a chemical cross linking of the two materials that resists separation by mechanical, chemical or thermal means. PA1 4. In the case of pistons of the invention, there is no need for a plurality of perforations as taught by Yanagi. PA1 5. The phenolic molding compound skin containing carbon fiber may extend over multiple surfaces of the molded plastic article.
Both items 1 and 2 can contribute to cracks, oxidation, and loss of strength.
U.S. Pat. No. 4,449,447 (Yanagi) discloses bonded-on heat resistant rings which can separate from the phenolic piston body on exposure to heat. Separation appears to be due to differential thermal expansion of the two materials. Other factors relating to separation appear to be post shrinkage of the phenolic material, and heat in combination with mechanical load.
Japanese Patent Application No. H3-236943 (1991) (Hokoi) discloses a compound molded product consisting of a pre-preg and a molding material of SMC, BMC or a pre-mix consisting of the same type of resin and fiber (substrate) in each material. The resultant molded article demonstrates improved, selective strength and good appearance. Unfortunately, the article does not appear suitable for use in a phenolic brake piston because there is no teaching in the Hokoi patent application of improved heat resistance at an acceptable price range for automotive brake usage.
Accordingly there is a need for a heat resistant phenolic plastic piston that resists separation when subjected to thermal, chemical or mechanical stress.
Transmission thrust washers, also known as thrust spacers, are currently manufactured from:
Thrust bearings are manufactured from needle bearings and assembly, and perform the same function as thrust washers.
The phenolic molding compound gives the lowest cost washer, but can degrade under heat and pressure and/or damage the friction plate it rides against.
The needle bearings are lower friction systems with higher costs.
Thrust washers have been combined with reactors or stators in the torque converters of automatic transmissions. The resulting articles have what is known as a castled surface and are molded or cast as one part. The invention disclosed herein is applicable to thrust washers in any of these applications.
Accordingly there is a need to produce an improved phenolic thrust washer with improved performance under high heat and pressure and reduced friction.